In a grinding mill, slurry flows from a mill shell chamber into pulp lifter chambers due to charge pressure and gravity as the mill shell rotates. Slurry is directed out of the mill (typically, via a central opening to a discharge trunnion) by pulp lifters or similar elements which define the pulp lifter chambers therebetween. As is well known in the art, each pulp lifter chamber is also partially defined by a mill grate, or discharge grate.
The pulp lifters typically are mounted on a discharge end wall (or mill head) of the mill. Often, the end wall is positioned at an angle (e.g., 75°) relative to a center line of the central opening in the end wall, i.e., the end wall forms a truncated cone. However, substantially vertical end walls are also common. As is known, a charge typically is positioned in a lower part of the mill shell chamber, filling the mill shell chamber to a limited extent.
As is known, the slurry flows into the pulp lifter chamber via apertures in the mill grate as the mill shell rotates. (For the purposes of discussion herein, rotation is assumed to be counter-clockwise, i.e., the discharge end, as viewed from inside the mill shell chamber, is assumed to rotate counter-clockwise. However, as is known, rotation may be clockwise or counter-clockwise.) In practice, slurry flows into a particular pulp lifter chamber under the influence of charge pressure and gravity when that chamber is between about the 8 o'clock and the 4 o'clock positions. As the mill shell rotates in a counter-clockwise direction, the particular chamber is raised from the 4 o'clock position upwardly to the 12 o'clock position, after which the chamber moves downwardly. As the chamber is so raised, and also as the chamber is lowered (i.e., after it has passed the 12 o'clock position), slurry flows from the chamber to the discharge trunnion.
Typically, the mill is rotated at a relatively high speed, to achieve optimal throughput. For example, a typical mill with an internal diameter of about 32 feet (approximately 9.8 meters) may rotate at about 10 revolutions per minute. Any decrease in rotation speed is understood to be counterproductive, as any such decrease would also decrease throughput, as is well known in the art.
In the prior art, attempts to increase production (i.e., mill throughput) have focused on increasing the sizes and/or the numbers of the apertures in the mill grates (or discharge grates). The idea is that a grate having larger apertures, and/or more apertures, should result in a larger volume of slurry flowing through the grate, and therefore into the pulp lifter chamber from the mill shell chamber, in a certain time period.
However, this assumes—incorrectly—that all the slurry in the pulp lifter chamber is moved out of the mill via the discharge trunnion. As is known, in practice, a portion of the slurry typically flows back into the mill shell chamber via the apertures in the mill grate as the mill shell rotates. Depending on the circumstances, the “back flow” may be relatively large. Typically, back flow of the slurry from a particular pulp lifter chamber occurs when that chamber is between about the 3 o'clock position and about the 9 o'clock position. Back flow generally is a more significant problem in mills with inclined discharge end walls.
It is clear that back flow has a negative impact on mill productivity, and it is also clear that back flow may have a very significant negative impact (especially where the end wall is inclined), depending on its volume. In any event, back flow clearly undermines attempts to increase mill productivity which are sought to be achieved solely by increasing the sizes and/or the numbers of the apertures in mill grates.